Starting low wattage (20w to 70w) or even higher wattage high intensity discharge lamps often requires the use of glow bottles in addition to a ballast that supplies a high voltage pulse to start the lamp. The glow bottles that have been suggested and employed contain a partial pressure (&lt;1 atmosphere) of argon or nitrogen or other gas mixtures. They may even include a partial pressure of mercury. These glow bottles contain an additional electrical lead that facilitates the "glow" or ionization of the contained gases when a sufficient potential is applied thereto. The glass vessel of the glow bottle must also be in close proximity to a lead-in of the opposite potential for the glow or ionization to occur.
The use of glow bottles works well in normal environments; however, their use becomes a problem as the size of the lamps gets smaller. For example, there is insufficient room in the neck region of a PAR20 lamp jacket to permit glow bottle placement. Also, such placement would encounter additional problems associated with distortion of the projected image or light beam if the glow bottle were placed in an area that interferes with the reflector surface or with the lensing optics of the lamp.
One solution to the problem that has been suggested employs radioactive krypton 85 as the arc tube gas fill; however, that solution requires the expense of extra filtering and hazardous material licenses.